Cigarette packing machines are known to feature a step-operated or continuous wrapping wheel having a number of peripheral seats, each for receiving a respective group of cigarettes--normally comprising twenty cigarettes arranged in three layers--which is fed to a respective seat on the wrapping wheel from a respective seat on a transfer wheel.
Before the group of cigarettes is transferred between the seats on the two wheels, a sheet of foil is fed between the wheels along the transfer path of the group from one wheel to the other, so that, when transferring the group, the sheet is inserted inside the respective seat on the wrapping wheel and simultaneously folded into a U about the group.
U.S. Pat. No. 5,392,586 describes a continuous wrapping unit operating according to the above known method, but which is complex in design and therefore expensive to produce. That is, the continuous movement of the wheels and the continually-changing positions of the seats with respect to each other and to the sheet of wrapping material call for complex mechanical solutions to keep the seats facing each other, and for synchronously inserting the sheet of wrapping material between the seats.
Moreover, regardless of whether the machine is operated continuously or in steps, the above method fails to provide for effectively folding the sheet of foil, on account of the increasingly fast operating speeds of modern packing machines--by now capable of producing over ten packets a second--reducing the folding time per sheet to a few hundredths of a second, and so stressing the sheet as to result in premature partial to total tearing of the tear-off portion.
GB patent application No. 2138382 discloses a continuos cigarette packing machine having a conveyor supporting a plurality of seats, each of which is capable of receiving and conveying a relevant sheet of wrapping material and a respective group of cigarettes. The conveyor feeds each seat through a first loading station, in which the seat receives the relevant sheet of wrapping material, and a successive second loading station, in which the seat receives the relevant product. Each sheet of wrapping material is fed to the relevant seat by a respective folding spindle, which is fed through an output of a line for supplying wrapping material to pick up and fold the sheet of wrapping material at least partly into a U. The folding spindle is then fed through the first loading station to be inserted into the relevant seat for feeding the sheet arranged in a substantially U-folded configuration inside the relevant seat.
In the known packing machine described above, perfect insertion of a folding spindle into a relevant seat for feeding the same with a relevant sheet of wrapping material is very difficult and generally involves some creeping of the external surfaces of the spindle with the internal surfaces of the seat; and this creeping may stress the sheet of wrapping material, which is clamped between the spindle and the seat, and result in a premature partial to total tearing of the tear-off portion.